The mammalian translational initiation machinery is a tightly controlled system that is composed of eukaryotic initiation factors, and which controls the recruitment of ribosomes to mediate cap-dependent translation. Accordingly, the mTORC1 complex functionally controls this cap-dependent translation machinery through the phosphorylation of its downstream substrates 4E-BPs and S6Ks. It is generally accepted that rapamycin, a specific inhibitor of mTORC1, is a potent translational repressor. Here we report the unexpected discovery that rapamycin's ability to regulate cap-dependent translation varies significantly among cell types. We show that this effect is mechanistically caused by rapamycin's differential effect on 4E-BP1 versus S6Ks. While rapamycin potently inhibits S6K activity throughout the duration of treatment, 4E-BP1 recovers in phosphorylation within 6 h despite initial inhibition (1-3 h). This reemerged 4E-BP1 phosphorylation is rapamycinresistant but still requires mTOR, Raptor, and mTORC1's activity. Therefore, these results explain how cap-dependent translation can be maintained in the presence of rapamycin. In addition, we have also defined the condition by which rapamycin can control cap-dependent translation in various cell types. Finally, we show that mTOR catalytic inhibitors are effective inhibitors of the rapamycin-resistant phenotype.cap-dependent translation ͉ mTORC1 ͉ rapamycin resistance T he mammalian translational initiation machinery governs the recruitment of ribosomes to mRNA to commence the production of protein synthesis. This machinery consists of various eukaryotic initiation factors (eIFs) that tightly regulate protein synthesis based on environmental cues. Importantly, initiation is an important step for cellular control because it is the ratelimiting step of translation (1).Two predominant pathways translate mammalian mRNA through cap-dependent and independent mechanisms. The capping of the 5Ј end of mRNA by m 7 GTP allows the recruitment of the eIF4F complex, eIF3, and the 40S ribosomal subunit to the 5Ј mRNA cap. Capindependent translation is mediated by an internal RNA structure called internal ribosome entry site (IRES), which recruits the ribosome independent of both the cap and the entire eIF4F complex (2).The initiation of cap-dependent translation is tightly regulated by extracellular conditions including glucose, nutrient, and growth factor levels. These factors control cap-dependent translation by regulating the evolutionarily conserved mTORC1 (mTOR, Raptor, mLST8) pathway (3). Activation of mTORC1 positively stimulates mRNA translation via its downstream substrates S6Ks and 4E-BP1/ eIF4E (4-7). Phosphorylation of 4E-BP1 by mTORC1 results in its dissociation from eIF4E, promoting assembly of the eIF4F complex. It is thought that S6K1 can phosphorylate translational regulators such as eIF4B and PDCD4 to enhance the translational efficiency of mRNAs with highly structured 5Ј UTRs (8-10).Therefore, growth factors positively regulate cap-dependent translation via mTORC...

Summary Proliferating mammalian cells use glutamine as a source of nitrogen and as a key anaplerotic source to provide metabolites to the tricarboxylic acid cycle (TCA) for biosynthesis. Recently, mTORC1 activation has been correlated with increased nutrient uptake and metabolism, but no molecular connection to glutaminolysis has been reported. Here, we show that mTORC1 promotes glutamine anaplerosis by activating glutamate dehydrogenase (GDH). This regulation requires transcriptional repression of SIRT4, the mitochondrial-localized sirtuin that inhibits GDH. Mechanistically, mTORC1 represses SIRT4 by promoting the proteasome-mediated destabilization of cAMP response element binding-2 (CREB2). Thus, a relationship between mTORC1, SIRT4 and cancer is suggested by our findings. Indeed, SIRT4 expression is reduced in human cancer, and its overexpression reduces cell proliferation, transformation and tumor development. Finally, our data indicate that targeting nutrient metabolism in energy-addicted cancers with high mTORC1 signaling may be an effective therapeutic approach.

Summary The mTORC1 signaling pathway integrates environmental conditions into distinct signals for cell growth by balancing anabolic and catabolic processes. Accordingly, energetic stress inhibits mTORC1 signaling predominantly through AMPK-dependent activation of TSC1/2. Thus, TSC1/2-/- cells are hypersensitive to glucose deprivation and this has been linked to increased p53 translation and activation of apoptosis. Herein, we show that mTORC1 inhibition during glucose deprivation prevented not only the execution of death, but also induction of energetic stress. mTORC1 inhibition during glucose deprivation decreased AMPK activation and allowed ATP to remain high, which was both necessary and sufficient for protection. This effect was not due to increased catabolic activities such as autophagy, but rather exclusively due to decreased anabolic processes, reducing energy consumption. Specifically, TSC1/2-/- cells become highly dependent on glutamate dehydrogenase-dependent glutamine metabolism via the TCA cycle for survival. Therefore, mTORC1 inhibition during energetic stress is primarily to balance metabolic demand with supply.

Summary The metabolism of glucose and glutamine, primary carbon-sources utilized by mitochondria to generate energy and macromolecules for cell growth, is directly regulated by mTORC1. We show that glucose and glutamine, by supplying carbons to the TCA cycle to produce ATP, positively feed back to mTORC1 through an AMPK-, TSC1/2-, and Rag-independent mechanism by regulating mTORC1 assembly and its lysosomal localization. We discovered that ATP-dependent TTT-RUVBL1/2 complex was disassembled and repressed by energy depletion, resulting in its decreased interaction with mTOR. The TTT-RUVBL complex was necessary for the interaction between mTORC1 and Rag, and formation of mTORC1 obligate dimers. In cancer tissues, TTT-RUVBL complex mRNAs were elevated, and positively correlated with transcripts encoding proteins of anabolic metabolism and mitochondrial function - all mTORC1-regulated processes. Thus, the TTT-RUVBL1/2 complex responds to the cell’s metabolic state, directly regulating the functional assembly of mTORC1, and indirectly controlling the nutrient signal from Rags to mTORC1.

DNA vaccines are a promising technology for the induction of Ag-specific immune responses, and much recent attention has gone into improving their immune potency. In this study we test the feasibility of delivering a plasmid encoding IL-15 as a DNA vaccine adjuvant for the induction of improved Ag-specific CD8+ T cellular immune responses. Because native IL-15 is poorly expressed, we used PCR-based strategies to develop an optimized construct that expresses 80-fold higher than the native IL-15 construct. Using a DNA vaccination model, we determined that immunization with optimized IL-15 in combination with HIV-1gag DNA constructs resulted in a significant enhancement of Ag-specific CD8+ T cell proliferation and IFN-γ secretion, and strong induction of long-lived CD8+ T cell responses. In an influenza DNA vaccine model, coimmunization with plasmid expressing influenza A PR8/34 hemagglutinin with the optimized IL-15 plasmid generated improved long term CD8+ T cellular immunity and protected the mice against a lethal mucosal challenge with influenza virus. Because we observed that IL-15 appeared to mostly adjuvant CD8+ T cell function, we show that in the partial, but not total, absence of CD4+ T cell help, plasmid-delivered IL-15 could restore CD8 secondary immune responses to an antigenic DNA plasmid, supporting the idea that the effects of IL-15 on CD8+ T cell expansion require the presence of low levels of CD4 T cells. These data suggest a role for enhanced plasmid IL-15 as a candidate adjuvant for vaccine or immunotherapeutic studies.

The mTORC1 signaling pathway is a critical regulator of cell growth and is hyper activated in many different cancers. Rapamycin, an allosteric inhibitor of mTORC1, has been approved for treatment against renal cell carcinomas and is being evaluated for other cancers. Mechanistically, mTORC1 controls cell growth in part through its two well-characterized substrates S6K1 and 4E-BP1. In this review, we discuss the implications of a recent finding that showed differential inhibition of S6K1 and 4E-BP1 by rapamycin, leading to cell-type-specific repression of cap-dependent translation. We discuss potential mechanisms for this effect, and propose that mTOR-specific kinase inhibitors, instead of rapamycin, should be considered for mTOR-targeted cancer therapy.

West Nile virus (WNV) is a member of the Flaviviridae family of vector-borne pathogens. Clinical signs of WNV infection include neurologic symptoms, limb weakness, and encephalitis, which can result in paralysis or death. We report that the WNV-capsid (Cp) by itself induces rapid nuclear condensation and cell death in tissue culture. Apoptosis is induced through the mitochondrial pathway resulting in caspase-9 activation and downstream caspase-3 activation. Capsid gene delivery into the striatum of mouse brain or interskeletal muscle resulted in cell death and inflammation, likely through capsid-induced apoptosis in vivo. These studies demonstrate that the capsid protein of WNV may be responsible for aspects of viral pathogenesis through induction of the apoptotic cascade.

Adherens junctions and desmosomes are responsible for mechanically coupling myocytes in the heart and are found closely apposed to gap junction plaques at the intercalated discs of cardiomyocytes. It is not known whether loss of cardiac gap junctions, such as described in cardiac disease states, may influence the expression patterns of other intercalated disc-associated proteins. We investigated whether the major cardiac gap junction protein connexin43 (Cx43) may be responsible for regulating adherens junctions,desmosomes and their associated catenins, in terms of abundance and localization at the intercalated discs of cardiomyocytes. In order to study the effect of loss of cardiac gap junctions on the intercalated disc-associated proteins, we used a combination of immunoblotting,immunofluorescence with confocal microscopy and electron microscopy to evaluate heart tissue from mice with cardiac-specific conditional knockout of Cx43. We found that the cardiac adherens junctions, desmosomes and their associated catenins, as well as vinculin and ZO-1, maintain their normal abundance, structural appearance and localization in the absence of Cx43. We conclude from these data that Cx43 is not required for the organization of the cell adhesion junctions and their associated catenins at the intercalated disc in the adult cardiac myocyte.